MACHINE TOOLS

MACHINE TOOLSComputer Integrated Manufacturing

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Submitted To: Sir. Zahid ShahSubmitted by: Ms. Pakeeza Bukhari, Roll # 17Ms. Sara Rehman, Roll # 25BSIEM- 6th semesterDated: 13 Sep, 2009

MACHINE TOOLS

Q # 1: Why are machine tools so important to our society? Machining has become indispensable to the society. It is used directly or indirectly in the manufacture of almost all the goods services being created all over the world. It is the basis of every thing manufactured such as sewing machines, paper, clothes, computers, space crafts, cars etc. Wherever metal is used in any man-made object, one can be sure that it must have reached its final stage through processing with machine tools. Even the parts made from plastics require metal dies made by machining processes. In short, machine tools have a great role in today’s progress.

Q # 2: What is the difference between tool and machine tools?

The differences are stated below:

Tools Machine Tools Tool is a portable device Tool is a non-powered device Tool can only powered by humans

Examples:Hammer, wrench, saws and shovel, pens, pencils and knives are tools

It is a stationary device It is a powered device It is powered by a power source or by

people if properly setup.

Examples:Lathes, shapers, planers, power drills or drill presses, milling machines, grinding machines, power saws, and presses (e.g., punch presses).

Q # 3: How is the size of following machine tools designated?The lathe:The size of lathe is designated by Distance between centers, Swing over the bed, Swing over the cross slide, Horse power of the motor, Range of spindle speeds, Range of feeds, Space occupied by the machine, Maximum diameter and length of the jobs that can be accommodated.

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Milling machine:The size of milling machine is designated by Size of the work table, Range of travels of the table in X-Y-Z direction, its Type: ordinary or swiveling bed type, Arbor size (diameter), Power of the main drive, Range of spindle speed, Range of table feeds in X-Y-Z directions and Floor space occupied.

The Shaper:The size of shaper is specified by the maximum length of stroke or cut it can make. Usually the size of shaper ranges from 175 to 195 mm. Besides the length of stroke, other particulars, such as type of drive, floor space required, length, width and depth of the machine, cutting to return ratio, number and amount of feed, power input etc are sometimes required for the complete specification of the shaper.

Drill Press:The size of a drill press is generally given as distance from the edge of the column to the center of the drill press spindle. For example on a 10 inch drill press, it would be possible to drill a hole 10 inch from the edge of the work piece. The vertical capacity is measured when the head is at the highest position and table is at the lowest position. Besides these, other particulars like maximum drill size that can be used, range of spindle speeds, range of feeds, power of main drive, floor space required by the machine etc are sometimes required for the complete specification of drill press.

Q #5: Draw sketches of following machine tools and label different parts. Also indicate the movement of different parts in different directions

a. Latheb. Milling machinec. The shaperd. Drill press

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Q # 5: List and define various operations that can be performed on machine tools mentioned in question no 3 and 4.

Lathe operations:

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Turning – produces straight, conical, curved, or grooved work pieces Facing – produces a flat surface at the end of the part Boring – to enlarge a hole Drilling - to produce a hole Cutting off – to cut off a work piece Threading – to produce threads Knurling – produces a regularly shaped roughnessThe operations are shown in figure also:

Milling operations:

Milling operations may be classified under four general headings as follows:

Face milling: Machining flat surfaces which are at right angles to the axis of the cutter.

Plain or slab milling: Machining flat surfaces which are parallel to the axis of the cutter.

Angular milling: Machining flat surfaces which are at an inclination to the axis of the cutter.

Form milling: Machining surfaces having an irregular outline.

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Special Operations :

Explanatory names, such as sawing, slotting, gear cutting, and so forth have been given to special operations. Routing is a term applied to milling an irregular outline while controlling the workpiece movement by hand feed. Grooving reamers and taps is called fluting. Gang milling is the term applied to an operation in which two or more milling cutters are used together on one arbor. Straddle milling is the term given to an operation in which two milling cutters are used to straddle the workpiece and mill both sides at the same time. Side milling is used to machine a vertical surface on the sides or the ends of a workpiece.

Fig: milling operations

Shaper Operations:It is a machine tool which uses single-point cutting tool to machine flat surfaces.

Machining horizontal surfaces:

Shaper is mostly used for machining flat surfaces of work-piece held on vise.7

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For horizontal surfaces, tool & work-piece are held tightly on tool holder and table respectively. Tool is made to machine total work-piece.

Machining vertical surfaces:

Shaper is also used to machine vertical surface. For this purpose, Vertical cut is made at machining end of work-piece, squaring

up a block and Surface of work-piece is properly aligned with vertical axis of ram.

Machining of angular surfaces:

Shaper makes angular cut at any angle other than right angle to horizontal. For this purpose, Vertical slide is swivelled to the required angle.

Machining slots and keyholes :

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With suitable tool, shaper can cut slot and keys on internal and external parts. For external keyway, a hole of size larger than keyway is cut at the end of

keyway for clearance of the tool (over travelling).

Machining irregular surfaces :

Shaper can produce convex or concave surface or combination of both. Forming tool is used and Round nose tool is used for irregular surface.

Machining splines or cutting gears :

Using index centre, a gear or equally spaced splines can be cut on work-pieces. Work is mounted between centres and spline is cut similar to a keyway. After every spline the work is rotated at a predetermined angle using index

plate. While cutting gears forming tool is used.

Drill press operations:

Drilling:It is a machining operation used to create a round hole in a workpart.

Reaming:Reaming is used to slightly enlarge a hole, to provide a better tolerance on its diameter, and to improve its surface finish. The tool is called a reamer, and usually it has straight flutes.

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Boring:It is an operation used for enlarging an en existing hole. It is an internal turning operation.

Tapping:This operation is performed by a tap and is used to provide iternal screw threads on an existing hole.

Counterboring:It provides a stepped hole, in which a larger diameter follows a smaller diameter partially into the hole.

Countersinking:This is similar to counterboring, except that the step in the hole is cone-shaped for flat headscrews and bolts.

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Q6: Define the cutting speed and feed for the following.The Lathe:Cutting Speed:Cutting speed is how fast the metal comes into contact with the tool at the cutting point. On a lathe, it is the rate at which the surface of the job passes the cutting tool. This takes into account the diameter of the job.The general formula for a cutting speed is as follows:

Feed:The feed of a lathe may be defined as the distance the cutting tool advances along the length of the work for every revolution of the spindle.

Milling Machine:Cutting Speed:Cutting speed is how fast the metal comes into contact with the tool at the cutting point. On a milling machine, it is the rate at which the milling cutter moves past the stock. This takes into account the diameter of the cutter.The general formula for a cutting speed is as follows:

Feed:Milling machine feed may be defined as the distance in inches (or millimeters) per minute that the work moves into the cutter. It is measured in in./min or mm/min.The formula to calculate feed for a milling machine is:Feed = no. of cutter teeth x chip/tooth x cutter r/minFeed (in./min) = N x CPT x r/minwhere,N = number of teeth in the milling cutterCPT = chip per tooth for a particular cutter and metal, as given in tabler/min = Revolutions per minute of the milling cutter

Drill Press:Cutting Speed:It is the distance that a point on the circumference of a drill will travel in 1 min.The general formula for a cutting speed is as follows:

where, r/min = Revolutions per minuteD = the diameter of the drill being used

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Feed:Feed is the distance that a drill advances into the work for each revolution. It may be expressed in decimals, fractions of an inch, or millimeters.

Q # 7: State different parts and features that are common in most of machine tools.Common parts and features:

Electric Motor Transmission Mechanism Spindle Base or Bed Slides or Ways Cutting Tool Mechanisms for different movements Different Levers, Hand wheels & Gauges Work piece holding device Accessories Cutting Speed Feed Rate

Q # 8: The base of machine tools is generally made of cast iron. State the advantages of using this material.

Advantages of using cast iron:

Good strength to weight ratio Typically lower cost than competing materials and relatively low cost per unit of

strength than other materials Lower density and higher thermal conductivity than steels at comparable tensile

strength levels Excellent machinability, allowing for high speeds and feeds and reduced (minimal)

energy due to the presence of free graphite Many iron castings can be used without heat treatment (as-cast) but, when needed, can

be heat treated to enhance overall properties or localized properties such as surface hardness

Excellent damping capacity Chemical analysis can be modified to provide improved special properties such as

corrosion resistance, oxidation resistance, wear or abrasion resistance, etc. Reduced tendency toward residual stresses and warpage than some competitive

materials Capability of casting with inserts of other materials

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Q # 9. The base of machine tools is generally provided with holes. What is the use of these holes?The base has holes so that it may be bolted to a table or bench/ ground.

Q #10. Why are tables on different machine tools provided with slots?The slots on tables are provided for direct mounting of the workpiece, vise or other fixture.

Q # 11. List different taper shank tools and accessories used on machine tools. Also state the advantage of making the shank tapered.Taper shank tools & accessories:Following are some of the taper shank tools and accessories used on machine tools

Tapered shank twist drill Tapered shank milling arbor Tapered shank dead center Tapered shank helical flute chucking reamer Taper shank straight flute jobber’s reamer Taper shank reamer arbor The Taper Shank Sleeve The Morse taper socket Extension

The advantage of making the shank tapered is that tapered shanks have more driving power and rigidity then straight shanks. Also tapered shanks have capacity to align and hold machine parts and to realign when they are repeatedly assembled and disassembled. This repeatability assures that tools such as centers in lathes, taper shank in drill presses, and arbors in milling machines will run in perfect alignment when placed in the machines.

Q12: list different types of slides or ways that are used in machine tools. GUIDEWAYS:

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1. Slide WaysSlide way shapes can be broadly classified as:

2. Antifriction WaysAntifriction Way shapes can be broadly classified as:

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Ball Type Open Roller Type Open Ball Type Closed Roller Type Closed

Flat Slide Ways Vee Slide Ways Dovetail Slide Ways Cylindrical

Slide Ways

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3. Frictionless Ways

Q # 13. The two most commonly used power transmission mechanisms in machine tools are belt and pulley, and gears. State the features of both mechanisms.Belt and pulley systems:

Belt & Pulley systems are used when there is a need to transmit rotary motion. The diagram below shows a simple system comprised of two pulley wheels and a belt. It

is a simple mechanical device to winch up and down a rope. When the motor is turned on it revolves the driver pulley wheel. The belt causes the driven pulley wheel to rotate as well, winding out the rope.

Pulley wheels are grooved so that the belt cannot slip off. Also, the belt is pulled tight between the two pulley wheels (in tension). The friction caused by this means that when the driver rotates the driven follows.

Most pulley wheels have a central shaft on which they rotate.

The amount of power transmitted through belts depends on following factors:15

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The velocity of the belt The tension under which belt is placed on the pulleys The arc of contact between belt & smaller pulley

Gears drive systems: The slipping of belts is a common phenomenon, in the transmission of power or motion

between two shafts. The affect of slipping is to reduce the velocity ratio of the systems. In various precision machines, in which velocity ratio is of definite importance, the only positive drive is by means of gears or toothed wheels.

To understand the gear system, consider two gears A and B mounted on shafts. The gears shown below are called spur gears because they mesh together. Gear ‘A’ is called the ‘driver’ because this is turned by a motor. As gear ‘A’ turns it meshes with gear ‘B’ and it begins to turn as well. Gear ‘B’ is called the ‘driven’

A gear drive is also provided when the distance between drivers and driven is very small.Some other features:

It transmits exact velocity ratio. It may be used to transmit large power. It may be used for small center distances of shafts It has high efficiency It has reliable service It has compact layout

Q # 14. List different work piece and cutting tool holding devices for each of the machine tools mentioned in question no 3.The lathe:Work pieces can be held by various methods

Work piece mounted between centers Work piece mounted within a single chuck

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Work piece mounted within a collet Work piece mounted on a faceplate

Workpiece Holding Devices: Lathe centers Lathe dog Three Jaw chuck Four Jaw chuck Magnetic chuck Face plates Mandrels Collet chuck Steadyrest Follower rest

Cutting Tool Holders: Toolholders for the Lathe Carriage: Quick-change tool post, dovetail type Turning toolholder Threading toolholder Drill toolholder Boring toolholder Tailstock turret Quick change cutoff toolholder Quick change knurling and facing toolholder A four-tool turret toolholder (it can hold facing, turning, threading or boring

tools)

Toolholders for the Lathe Tailstock: Drill chuck Tailstock turret Standard-type tool post Straight shank toolholder Right hand toolholder for carbide tool bits Left hand toolholder Right hand toolholder Cutoff toolholders (R.H offset, straight, L.H offset) Boring bar toolpost (light boring toolholder, heavy-duty boring bar toolholder,

medium boring toolholder)

Milling machine:

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Work pieces can be held by various methods Clamping Workpieces to the Table Clamping a Workpiece to the Angle Plate Clamping Workpieces in Fixtures Holding Workpieces Between Centers Holding Workpieces in a Chuck Holding Workpieces in the Vise

Workpiece Holding Devices: Vise Parallels Fixtures Adjustable angle plate Rotary table or circular milling attachment Collets

Cutting Tool Holders: End mill adapters Spindle adapters Chuck adapter Quick change tooling High-speed milling attachment Vertical spindle attachment Universal milling attachment Offset boring head Offset boring head and tools

Drill press:Tool holding devices for drill press:The drill press spindle provides a means of holding & driving the cutting tool. It may have a tapered hole to receive taper shank tools or threaded for mounting drill chuck. Although there is a variety of tool holding devices and accessories, the most common used in machine shop are drill chucks and drill shank adapters called sleeves and stock etc. Work holding devices for drill press:Because of great force applied by the machines in drilling, some means must be provided to keep the workpiece from turning with the drill or from climbing up the flutes after the drill breaks through. This is necessary not only for safety‘s sake but also for workpiece rigidity and good workmanship.Some of the workholding devices are:

Strap clamps and T-bolts

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Angle plates Drill press vises Vee blocks Jigs & fixtures

The shaper:Job holding devices for shaper:Most of the jobs on a shaper are held in a vice. But often they are clamped to the table also.

Vices:Following are the types of vices used on shaper. Plain or parallel vice Swivel machine vise The vertical vice The precision angle vice The universal machine vice

Work bolted directly to table:Work is bolted to the table using following techniques. Using T-bolts Using Stop pins Using Stop pins & toe dogs Using Stop pins & table strip Angle plates

Tool holders for shaper:The tool holders for shaper for shaper are:

Adjustable shaper tool holder Extension shaper tool holder

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Q # 15: List different mechanisms used for the movement of different parts of machine tools. State characteristics of each.Mechanical Drive Mechanisms:

1. A rack and pinion is a pair of gears which convert rotational motion into linear motion. The circular pinion engages teeth on a flat bar - the rack. Rotational motion applied to the pinion will cause the rack to move to the side, up to the limit of its travel. This arrangement provides a lesser mechanical advantage than other mechanisms.

2. A ball screw is a mechanical device for translating rotational motion to linear motion. A threaded shaft provides a spiral raceway for ball bearings which act as a precision screw. As well as being able to apply or withstand high thrust loads they can do so with minimum internal friction. They are made to close tolerances and are therefore suitable for use in situations in which high precision is necessary. The ball assembly acts as the nut while the threaded shaft is the screw.

3. A Belt is a looped strip of flexible material, used to mechanically link two or more rotating shafts. They may be used as a source of motion, to efficiently transmit power, or to track relative movement. Belts are looped over pulleys. In a two pulley system, the belt can either drive the pulleys in the same direction, or the belt may be crossed, so that the direction of the shafts is opposite.

Hydraulic Drive Mechanism:A hydraulic or hydrostatic drive system or hydraulic power transmission is a drive or transmission system that uses hydraulic fluid under pressure to drive machinery. The term hydrostatic refers to the transfer of energy from flow and pressure, not from the kinetic energy of the flow.Hydraulic cylinders (also called linear hydraulic motors) are mechanical actuators that are used to give a linear force through a linear stroke. A hydraulic cylinder is without doubt the best known hydraulic component. Hydraulic cylinders are able to give pushing and pulling forces of millions of metric tons, with only a simple hydraulic system.

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Q # 16. What are the generally used cutting tools materials and what properties do they impart to the cutting tool?Tool materials:The various tool materials used in today's manufacturing operations are high-carbon steel, high-speed steel, cemented carbides, ceramics, diamond, and cubic boron nitride (CBN)

High carbon steels:High carbon steels are oil- or water-hardened plain carbon steels with .9 to 1.4 percent carbon content. They are used for hand tools such as files and chisels, and only to a limited extent for drilling & turning tools.They impart such properties to tools made from them that such tools maintain a keep edge & can be used for metals that can be used for such metals that produce low tool-chip interface temperatures-for example, aluminum, magnesium, copper, and brass. These tools, however, tend to soften at machining speeds above 50 feet per minute (fpm) in mild steels.

High-speed steel:High-speed steel may be used at higher cutting speeds (100 fpm in mild steels) without losing their hardness.High-speed steel is some times used for lathe tools when special tool shapes are needed, especially for boring tools. However, high-speed steel is extensively used for milling cutters. These cutters usually have a longer working life.

Cemented carbides:A carbide, generally, is a chemical compound of carbon and a metal. The term Carbide is commonly used to re-present to cemented carbides, the cutting tools composed of tungsten carbide, titanium carbide, or tantalum carbide & cobalt in various combinations. A typical composition of cemented carbide is 85 to 95 percent of tungsten & the remainder a cobalt binder for the tungsten carbide powder.Cemented carbides are extremely hard tool materials (above RA 90), have a high compressive strength & resist wear & rupture. Coated carbide inserts are often used to cut hard or difficult-to-machine work pieces. Titanium carbide (TiC) coating offers high wear resistance at moderate cutting speeds and temperatures. Aluminum oxide (Al2O3) coating has high resistance to crater wear and reduces friction between the tool face and the chip, thereby reducing the tendency for built-up edge.Cemented carbides are the most widely used tool materials in the machining industry. They are particularly useful for cutting tough alloy steels, which quickly break down high-speed tool steels.

Ceramic tools:Ceramic or “cemented oxide” tools are made primarily from aluminum oxide. Some

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manufacturers add titanium, magnesium, or chromium oxides in quantities of 10% or less. The tool materials are molded at pressures over 4000 psi and sintered at temperatures of approximately 30000F. This process partly accounts for the high density and hardness of cemented oxide tools.Cemented oxides are brittle and require that machines and setups are rigid and free of vibration. Ceramic tools should be used as a replacement for carbide tools that are wearing rapidly, but not to replace carbide tools that are breaking. Cubic boron nitride:

CBN is next to diamond in hardness and therefore can be used to machine plain carbon steels, alloy steels, and gray cast irons with hardnesses of 45 Rc and above.CBN inserts consist of a cemented carbide substrate with an outside layer of CBN formed as an integral part of the tool. Tool life, finishes, and resistance to cracking and abrasion make CBN a superior tool material to both carbides and ceramics.

Diamond tools:

Industrial diamonds are sometimes used to machine extremely hard work poeces. Only relatively small removal rates are possible with diamond tools, but high speeds are used and good finishes are obtained.Diamond tools are particularly effective for cutting abrasive materials that quickly wear out other tool materials. Nonferrous metals, plastics, and some nonmetallic materials are often cut with diamond tools.

Q 17: Briefly describe the differences between conventional machine tools and CNC machine tools.

The differences are stated as follows:

CONVENTIONAL MACHINE TOOLS CNC MACHINE TOOLS

Machine tools can be operated manually, or under automatic control

They are operated automatically

It has a complex system of gears and levers to control the machine and the piece being worked on.

Used a series of numbers punched on paper tape or punch cards to control their motion.

Cutting feed rate and spindle speeds Cutting feed rate and spindle speeds 22

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are controlled by levers and handwheels.

are controlled through program instructions.

They can produce pieces which are not complex

They can produce much more complex pieces

These machines are semi-automatic These machines are highly automatic They has a limited capacity to hold

tools They have either turret or belt

toolholders which can hold more than 150 tools.

They have a limited variety to accomplish machining operations.

The advanced toolholders gives the machine a vast capability to accomplish many different machining operations.

Tool selection and changing is done manually

Tool selection and changing is under the program’s control

Time is consumed in manual operations

A little time is wasted in applying a tool to the job.

It has a less degree of dimensional accuracy as well as less production rate as compared to CNC machine tools.

It has a high degree of dimensional accuracy as well as high production rate.

Stepped drives are very common in conventional machine tools where a discrete number of speeds and feeds are available and preferably in geometric progression series.

They have stepless drives enabling optimum selection and flexibly automatic control of the speeds and feeds

References:Book sources:

Machine tool technology basics by Stephen F.Krar, Arthur gill, Peter smid Introduction to manufacturing processes & workshop technology by Rajendar Singh Machine tool practices by R. Kibbe, O. Meyer, E. Neely, and T. White Manufacturing processes by Harold V. Johnson A text book of machine design Work shop Technolgy by Hajra choudry Advances in Manufacturing Technology C.J Thomas Mechatronics by Ganesh S. Hegde

Website sources: www.engbasics.com

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www.efunda.com http://skilledtradesmath.com/files/Drill_Speeds_and_Feeds2.ppt http://www.eng.hmc.edu/E8/Other/Cutspeed.htm http://www.feedforward.com.au/machine_tools_sample.pdf www.ironcasting.com www.americanmachinetools.com www.wikipedia.org

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